Patent Document 1 discloses an acceleration sensor having a beam-structured movable portion. The sensor element portion of the acceleration sensor has a movable electrode as part of the beam structure, first and second fixed electrodes, and a fixed electrode for canceling electrostatic force. The sensor element portion is covered and protected by a recessed portion of a cap substrate, with electrode extraction portions connected to the four electrodes through interconnections and not covered by the recess of the cap substrate.
The beam structure of an acceleration sensor is formed by previously forming a sacrificial layer for supporting it, forming and patterning a conductive layer (e.g. doped polysilicon) on it, and then removing the sacrificial layer. Therefore, when an opening for making contact with the silicon substrate on which the beam structure resides is formed alter the formation of the conductive layer by applying photolithography from the same side as the beam structure, it is necessary that the photoresist satisfactorily cover a step height approximately equal to the sum of the thickness of the conductive layer and the thickness of the sacrificial layer. Now, the conductive layer itself has a thickness of about 3.5 to 4.0 μm and the sacrificial layer has a thickness of about 2.0 to 2.5 μm, and thus the sum of the two exceeds 5 μm. It is therefore desirable to form the photoresist thick, in order to satisfactorily cover such a step height.
However, in general, forming photoresist thick is likely to cause cracks. Therefore, in order to form thick photoresist without causing cracks, it is desirable to apply the photoresist in a plurality of coating steps. Also, using a thick photoresist as a mask for etching incurs a reduction in etching rate, which necessitates an increased number of etching steps.
Patent Document 2 discloses a method for forming a contact hole having a tapered portion in its upper part. Patent Document 3 discloses a technique for improving the step coverage of metal interconnections, in which plural kinds of interlayer films are stacked and planarized and a tapered contact hole is formed by utilizing differences among etching rates of the films, so as to facilitate provision of an upper-layer metal interconnection.
Patent Document 1: Japanese Patent Application Laid-Open No. 2001-119040
Patent Document 2: Japanese Patent Application Laid-Open No. 8-274066 (1996)
Patent Document 3: Japanese Patent Application Laid-Open No. 5-190690 (1993)